This invention relates to alternating current (AC) voltage-to-direct current (DC) voltage conversion circuits and particularly to a circuit for regulating input line voltages, that may vary in peak amplitudes, for providing a constant magnitude, minimal ripple amplitude supply voltage. Due to the difference between power distribution systems employed by different nations, applied line voltages may vary from as low as 90 volts, as used in Japan, to as high as 250 volts, as encountered in Europe. In the United States there is about a 20 percent variation of the nominal 120 volts on standard power transmission lines.
As a result, problems may be encountered by international travellers having small appliances, such as electric shavers, travel alarm clocks, or blenders, if the working voltage of the driving elements in such appliances is exceeded. In particular, some electric shavers contain a bulky AC wound motor that is used to drive the shaver heads. Presently, it is sometimes necessary to manually switch the shaver thereby tapping the correct proportion of the motor windings corresponding to the particular line voltage being used. The burden is on the user to see to it that this switching is properly performed in accordance with the different line voltages that may be encountered while travelling worldwide. If the traveller does not correct for the specific line voltages, the shaver may be destroyed by excessive currents.
The AC motor referred to in the previous paragraph also presents a production disadvantage in the manufacture of the electric shaver. AC motors are bulky and expensive by nature. It is therefore desired to use small, inexpensive direct current motors requiring minimum working voltage. An AC-to-DC regulator circuit capable of providing a high degree of regulation with varying amplitudes of input line voltages is required to facilitate the use of such DC motors.
Although present AC-DC regulators are adequate for regulating input voltages that have amplitudes that vary 10 to 20 percent, these regulators are not suitable to be used in small appliances in which the peak amplitude of the input line voltage varies over 100 percent. To maintain nominal working voltages of 100 volts in response to line voltages with amplitudes of 250 volts, present regulators generally have to conduct currents of large magnitudes. To withstand the resulting power dissipation, it is necessary to use high power semiconductor devices and heat sinking to provide protection to such devices. This undesirably increases the physical size of such regulators. Moreover, power devices are expensive and provide disadvantages in a marketing environment that demands minimal production costs.
Another problem common to most present regulators is that of transients. Transient voltages can occur during either connection or disconnection of the regulator from the line voltage supply. If the transient voltage causes a false trigger signal to occur simultaneously with the peak value of the applied line voltage, the output voltage of the regulator could exceed the rating of the motor and destroy it.
Thus, a need exists for a regulator circuit providing a constant output voltage and which is operable in environments in which extreme amplitude variances in line voltages occur and which does not require manual switching techniques to protect the circuit. A need also exists to develop a regulator circuit which is not damaged by false triggering in response to transients.